The electrocardiogram (ECG or EKG) is a diagnostic tool that is routinely used to assess the
electrical and muscular functions of the heart. The electrocardiogram can measure the rate
and rhythm of the heartbeat, as well as provide indirect evidence of blood flow to the heart
muscle.
The electrocardiogram translates the heart's electrical activity into line tracings on paper.
The spikes and dips in the line tracings are called waves.
• The P wave is a record of the electrical activity through the upper heart chambers
(atria).
• The QRS complex is a record of the movement of electrical impulses through the
lower heart chambers (ventricles).
• The ST segment shows when the ventricle is contracting but no electricity is flowing
through it. The ST segment usually appears as a straight, level line between the QRS
complex and the T wave.
• The T wave shows when the lower heart chambers are resetting electrically and
preparing for their next muscle contraction.
Heart Function and the ECG
Electrode leads on the chest wall are able to detect electrical impulses that are generated by
the heart.. By interpreting the tracing, the physician can learn about the heart rate and
rhythm as well as blood flow to the ventricles (indirectly).
Picture of rhythm strip showing a normal 12-lead ECG
Rate refers to how fast the heart beats. Normally, the SA node generates an electrical
impulse 60-100 times per minute. Bradycardia (brady=slow+cardia=heart) describes a heart
rate less than 60 beats per minute. Tachycardia (tachy=fast+cardia=heart) describes a heart
rate faster than 100 beats per minute.
Rhythm refers to the type of heartbeat. Normally, the heart beats in a sinus rhythm with
each electrical impulse generated by the SA node resulting in a ventricular contraction, or
heartbeat.
There are 8 steps method to interprete a rhythm strip:
• Step 1: Determine the rhythm
-For atrial rhythm, measure the P-P intervals
-To determine the ventricular rhythm measure the intervals between two consecutive R
waves in the QRS complexes.
• Step 2: Determine the rate
• Step 3: Evaluate the P wave
• Step 4: Measure the PR interval
• Step 5: Determine the QRS complex duration
• Step 6: Examine the T waves
• Step 7: Measure the QT interval duration
• Step 8: Check for ectopic beats and other abnormalities
What makes sinus rhythm normal?
1. Atrial and ventricular rhythms are regular.
2. Atrial and ventricular rates fall between 60 and 100 beats/minute, the SA node’s
normal firing rate, and all impulses are conducted to the ventricles.
3. P waves are rounded, smooth, and upright in lead II, signaling that a sinus impulse has
reached the atria.
4. The PR interval is normal (0.12 to 0.20 second), indicating that the impulse is
following normal conduction pathways.
5. The QRS complex is of normal duration (less than 0.12 second), representing normal
ventricular impulse conduction and recovery.
6. The T wave is upright in lead II, confirming that normal repolarization has taken place.
7. The QT interval is within normal limits (0.36 to 0.44 second).
8. No ectopic or aberrant beats occur.
There are a variety of abnormal electrical rhythms, some are normal variants and some are
potentially dangerous. Some electrical rhythms do not generate a heartbeat and are the
cause of sudden death.
Examples of heart rhythms include:
• Normal sinus rhythm
• Sinus tachycardia
• Sinus bradycardia
• Atrial fibrillation
• Atrial flutter
• Ventricular tachycardia
• Ventricular fibrillation
There can also be delays in transmission of the electrical impulse anywhere in the system,
including the SA node, the atria, the AV node, or in the ventricles. Some aberrant impulses
cause normal variants of the heart rhythm and others can be potentially life threatening.
Some examples include:
• 1st degree AV block
• 2nd degree AV block, type I (Wenckebach)
• 2nd degree AV block, type II
• 3rd degree AV block or complete heart block
• Right bundle branch block
• Left bundle branch block
There can also be short circuits that can lead to abnormal electrical pathways in the heart
causing abnormalities of rate and rhythm. Wolfe-Parkinson-White (WPW) syndrome is a
condition where an abnormal accessory pathway at the AV node causes tachycardia.
The ECG tracing can also provide information about whether the heart muscle cells are
conducting electricity appropriately. By analyzing the shape of the electrical waves, the
physician may be able to determine if there is decreased blood flow to parts of the heart
muscle. The presence of an acute blockage associated with a myocardial infarction or heart
attack can be determined as well. That's one of the reasons that an ECG is done as soon as
possible when a patient presents with chest pain.
Reasons to Have an ECG
The ECG is used to assess heart function. Patients who complain of chest pain or shortness of
breath will often have an ECG as one of the first tests to help determine if there is an acute
myocardial infarction or heart attack present. Even if there is no heart attack, the ECG can
help decide whether the pain is due to angina or narrowing of blood vessels to the heart
muscle (atherosclerosis). It is important to realize that an initial ECG may be normal even if
there is heart disease present. Serial EKGs may be needed over time to find an abnormality.
ECGs are often performed when a patient complains of lightheadedness, palpitations, or
syncope (passing out) since abnormal heart rate and rhythms may affect the heart's ability to
pump blood and provide the body with oxygen.
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